Human platelets were extacted with acid-ethanol and platelet-derived TGF-beta was purified from the extract by a two-column procedure using sequential gel filtration in the absence and then presence of urea. Purified TGF-beta is a protein of 25,000 daltons, and it is comprised of two 12,500 dalton subunits held together by disulfide bonds. The purified factor elicits its biological activity at concentrations less than 4pM. Comparative studies showed that platelets contain 100 times more TGF-beta than do other non-neoplastic tissues. Platelets also contain a peptide growth factor related to EGF. These two new growth factors can interact mechanistically. Incubation of TGF-beta with NRK cells for 6 h results in an increased number of cell surface EGF receptors. IGF-II receptors are not affected. Shorter incubations with TGF-beta show that this peptide can also increase the kd of the high affinity EGF receptor. TGF-beta has bifunctional effects on explant cultures of bovine vascular smooth muscle cells; it inhibits monolayer growth yet promotes growth in soft agar. TGF-beta is also produced by activated monocytes.